Known methods of bonding a ceramic to a metal involve the use of interlayer materials which either melt at the bonding temperature, such as a braze, or which involve special coating processes for the material surfaces to be bonded, such as pre-coating the surfaces with an activating material. In some methods of bonding, an interlayer material having a composition that approximates the composition of the initial metal bonding surface is utilized, such as disclosed by Lasater (U.S. Pat. No. 6,221,513). Lasater discloses a method for forming a hermetically sealed bond for use in implantable medical devices. Hill (U.S. Pat. No. 3,594,895) discloses another approach to forming a ceramic to metal seal. Fey, et al. (U.S. Pat. No. 6,521,350) discloses the use of pure nickel or nickel alloy to bond titanium to a ceramic, such as alumina or zirconia.
Cusano (U.S. Pat. No. 3,994,430) discloses a method of directly bonding metal to ceramic substrates where a very thin layer of an interlayer material is placed between the metal and the ceramic to be bonded. The system is heated in an inert atmosphere to a temperature between the eutectic temperature of the interlayer material and the melting point of the metal. Cusano focuses on bonding copper to a ceramic substrate, such as alumina or beryllia.
In one application, it is desired to have a platinum eyelet attached to a titanium alloy end cap that is in turn bonded to a hollow ceramic tube, which is implantable in living tissue as, for example, a microstimulator or microsensor. The BION of Advanced Bionics Corporation is one such device. The inventors have demonstrated that welding creates cracks at the platinum eyelet and the titanium alloy end cap weld joint. Low tensile strength has been measured in these welded joints. Continuous welding yielded an average strength of 4.5 lbf; double pass continuous welding yielded 4.5 lbf; and single shot welding yielded 16.2 lbf.
A hermetic, strong end cap that is bonded to a ceramic is needed, especially for implantable components.